The present invention relates to absorption spectroscopy, and more particularly to a method and apparatus for performing ring-down spectroscopy using a continuous wave light source and using two polarizations of light in a cavity.
Cavity ring-down spectroscopy (CRDS) is a general, high sensitivity technique for measuring absorption. CRDS has been primarily applied to the study of very weakly absorbing species or dilute species concentrations. In CRDS, monochromatic light from a laser is injected into a high finesse optical resonator, called a ring-down cavity (RDC), which encloses a sample. When the light source is abruptly terminated, light trapped inside the RDC decays due to finite resonator losses and can be monitored by detecting light transmitted through a mirror of the RDC. Typically, the light exiting the RDC decays exponentially in time with a decay time constant xcfx84, called the ring-down decay constant. The rate of decay, called the ring-down rate (RDR) is directly proportional to resonator losses due to transmission, scattering, diffraction, absorption etc., and absorption by sample species at a particular wavelength. The RDR is inversely proportional to xcfx84. A spectrum of the sample species is obtained by measuring the RDR as a function of wavelength.
CRDS has been performed using both pulsed and continuous wave (CW) laser sources. Pulsed CRDS (P-CRDS) suffers from several limitations. Because the pulse duration is typically less than several RDC round-trip times, no energy buildup can occur in the optical resonator. The RDC output is therefore severely attenuated at the cavity output. This attenuation produces weak output signals with inferior signal to noise characteristics. Furthermore, many pulsed laser sources have repetition rates lower than 1 kHz, which preclude real-time spectral acquisition and extensive averaging to improve signal-to-noise ratio. Furthermore, most pulse laser sources have line-widths exceeding hundreds of kHz for nanosecond pulses (even when Fourier-transform-limited), which limits the spectral resolution of the CRDS technique.
Recently, efforts have been made to overcome most of these limitations by the use of narrow band ( less than 10 MHz) CW lasers. By coupling the CW laser into the high finesse RDC, light inside the RDC is built up, and cavity throughput increases. In principle, cavity throughput can become close to 1.0, allowing shot-noise-limited detection, as demonstrated by Zare and co-workers in 1998. Much current laser ring-down spectroscopy is still performed with fairly costly laser sources, e.g. Ti:Sapphire lasers, optical parametric oscillators, and external cavity diode lasers. The advent of laser diodes as CW laser sources dramatically decreased the cost of CRDS based laser systems. Semiconductor laser diodes can potentially provide inexpensive laser sources for CW-CRDS as they rapidly improve in power, wavelength coverage, and reliability.
Diode lasers are CW sources with relatively weak output powers, e.g., a few milliwatts (mW). If a CW diode laser is modulated with a small duty cycle (i.e., shorter than the cavity roundtrip time), resonator interference effects and the need to frequency match the laser to a narrow cavity resonance, can be eliminated. Unfortunately, only very small cavity throughput can be achieved this way, which results in very noisy signals.
For example, a high finesse resonator constructed with 99.999% reflecting mirrors attenuates any input by about 10xe2x88x9210. The injection of a 1 mW pulse of non-mode-matched CW radiation would result in 10xe2x88x9213W of light power at the beginning of the ring-down decay waveform. Such a signal is virtually impossible to detect with a broadband photodetector, particularly in the infrared where photomultipliers are generally ineffective. To record ring-down decay transients with decay constants of order 1 microsecond, a 1 MHz bandwidth, 10xe2x88x9214 W/Hzxc2xd noise equivalent detector is typically necessary and a detector noise of about 10xe2x88x9211 is calculated. The noise, which is inherent in the detection process, is significantly larger than the ring-down signal power at any point in the waveform.
If the laser is locked to one of the cavity resonances over the course of several decay constants, and the laser linewidth is smaller than the cavity resonance, then substantial buildup of the intracavity field can occur. Consequently, strong ring-down signal can be observed after the laser beam is quickly terminated, i.e. faster than xcfx84. The cavity throughput may become close to 1.0, which allows shot-noise limited detection of the ring-down signal. Shot-noise-limited detection of several mW of light can produce high signal-to-noise ratios, on the order of 1,000,000:1.
Laser diode sources typically have linewidths broadened by high frequency jitter to about 10 MHz. This is significantly larger than the typical ring-down cavity linewidth of a few kHz. Classical error signal extraction is, therefore, extremely difficult, The problem of locking a laser and a super-cavity is illustrated in FIGS. 1a and 1b. FIG. 1a illustrates locking an ordinary low finesse cavity and laser together. In this case, the laser linewidth is much smaller than the cavity linewidth. Frequency modulation of the laser typically causes modulation of the intensity of light transmitted through the cavity. The amplitude of this intensity modulation, which represents a form of error signal, is generally proportional to the frequency detuning of the laser with respect to the cavity resonance frequency. The phase of the intensity modulation changes sign when the laser passes from a frequency less than the cavity resonance frequency to a frequency greater than the cavity resonance frequency. The error signal is zero if the laser line is centered on the cavity resonance frequency. If this error signal is demodulated, amplified and applied to the element that changes the laser frequency, the laser will be kept in resonance with the cavity.
When the same laser is locked to a very high finesse cavity, the effective laser linewidth is typically much larger than the cavity resonance frequency, as shown in FIG. 1b. The instantaneous frequency changes very rapidly over a spectral range several orders of magnitude larger than the laser linewidth. The duration of the laser center frequency changes can be as short as a few microseconds, so that the laser frequency changes essentially instantaneously. Because the response time of the a ring-down cavity depends on the bandwidth of its frequency changing element, typically a piezo-electric transducer (PZT), most cavities can respond at low kHz rates. In this case, no distinct error signal will be produced from this cavity and the xe2x80x9csimplexe2x80x9d servo-loop won""t work.
In principle, the signal error problem can be overcome using the Pound-Dever locking technique and feedback to an electro-optic or acousto-optic modulator to change the laser frequency of laser light reaching the ring-down cavity. However, these systems require extreme mechanical stability and, for practical systems, a very large locking bandwidth. Furthermore, if the laser light is extinguished, e.g., to measure the decay constant, the Pound-Dever lock would be lost. The locking servo becomes ineffective after each decay waveform measurement, which introduces long system recovery times. Furthermore, strong mechanical perturbations might cause the laser to re-lock to a cavity resonance separated in frequency from the previous resonance. At best, such a locked system would be intermittently usable for recording absorption lines of specific species in real-time, e.g. a concentration measurement every few seconds.
An alternative method for locking a laser diode (LD) to a high finesse cavity utilizes optical feedback. In this method, a small fraction of the laser radiation already accumulated inside the cavity is sent back to the diode laser. The distance between the LD and the cavity input mirror is adjusted so that the feedback radiation is in phase with the laser radiation. As a result, optical feedback takes place, which substantially reduces the LD linewidth. Ultimately, the laser linewidth can be reduced to much less than the cavity resonance linewidth. Substantial intensity also builds up inside the cavity. This works well with a relatively low finesse cavity when the LD is locked to the cavity and never turned off.
In CW-CRDS, the light source must be extinguished to measure the ring-down decay waveform. Consequently the optical feedback method encounters the same locking electronics problem as the Pound-Dever method. In the optical feedback case, a servo-loop needed to keep the feedback signal in phase with the laser radiation may be perturbed and driven out of phase thus converting positive feedback to negative feedback. Consequently, the system may re-lock onto an arbitrary cavity mode resulting in intermittent operation. Both the optical feedback system and Pound-Dever system must be mechanically stabilized with interferometric precision. Therefore, neither system can be easily implemented in environments having large mechanical perturbations. Furthermore, the optical feedback locking technique cannot be effectively used for CW sources other than LDs. Even under optical isolation, the optical feedback often results in phase fluctuations and mode hopping of the LD, so that the laser can not be relocked to the same cavity mode from shot to shot. These problems result in unreliable operation of CW-CRDS systems and preclude the demonstration of compact, LD-based, CW-CRDS systems.
There is a need, therefore, for a cavity ring down spectroscopy system that provides highly sensitive absorption measurements in an environment of serious external mechanical perturbations.
Accordingly, it is a primary object of the present invention to provide a spectroscopy tool that can be used for ultra-sensitive, real-time detection of samples inside optical cavities. It is a further object of the invention to provide a high finesse optical resonator that is excited by a CW single-frequency light source and optically isolated from the resonator. Another object of the invention is to provide a means to ensure that the cavity resonance follows the light source as the light source frequency is swept over an arbitrarily large spectral interval. It is an additional object of the invention to provide means to automatically recover after a strong mechanical perturbation, which may bring the center frequency of the cavity resonance far away from the actual light source frequency.
The above objects and advantages are attained by an instrument comprising a high finesse optical resonator, such as a ring-down cavity (RDC), a single-frequency CW light source (CWLS), a frequency shifter and first and second detectors. The CWLS produces input light having a first component with a first polarization and a second component having a second polarization. Suitable optics couple the input light into the resonator. The ring-down cavity typically has a first mirror and a second mirror. A translator controllably moves the first mirror. The ring-down cavity is optically isolated from the light source so that any light reflected from the cavity is precluded from perturbing the light source. The frequency shifter shifts a mean frequency of the first component with respect to a mean frequency of the second component by a frequency shift xcex94xcexd. The first detector is configured to measure an intensity of a signal beam with the first polarization exiting from the ring resonator. The second detector is configured to measure an intensity of a tracking beam having the second polarization exiting from the ring resonator. The frequency shift xcex94xcexd is equal to the difference between a first resonant frequency of the resonator corresponding to a first cavity mode with the first polarization, and a second resonant frequency corresponding to a second cavity mode having the second polarization.
A tracking circuit determines an oscillating voltage from a signal received from the second detector. The oscillating voltage drives the translator to oscillate the first mirror around a central value corresponding to a resonant coupling between said ring resonator and the first component of input light. A threshold detector delivers a trigger pulse to the frequency shifter when an intensity of the signal beam reaches a predetermined value. The trigger pulse causes the frequency shifter to temporarily change the frequency shift xcex94xcexd, thereby temporarily decoupling the first component of input light from the ring resonator.
The instrument measures the cavity time delay as a function of source wavelength to obtain an absorption spectrum of a medium inside the cavity. The system is insensitive to mechanical perturbations because the cavity does not provide any reflected light to the light source. The frequency stability and reproducibility of the light source determine the frequency stability and reproducibility of the instrument.
The instrument also includes means to ensure reliable and reproducible injection of light source radiation into the cavity. The instrument tracks cavity resonance as it is swept in frequency across the light source laser line. The tracking ensures that the cavity and light source frequencies are always close to each other.
An embodiment of the present invention includes a ring down spectroscopy method. The method couples radiation from a continuous-wave light source (CWLS) into a ring-down cavity. The radiation is swept in frequency by an RDC length, e.g., one or more free spectral ranges of the cavity, to excite one or more resonant modes of the cavity. A threshold detector is triggered when a fundamental mode of the cavity reaches a predetermined threshold value. The threshold detector then triggers a digitizer to sample a ring-down decay curve. A processor stores the points of a waveform generated from the decay curve. A point on an absorption spectrum can be determined by extrapolating a decay constant from a logarithm of the waveform points. A portion of an absorption spectrum can then be determined by iteratively repeating the method while tuning the CWLS over a given frequency or wavelength range.